How polydimethylsiloxane degrades in the
environment

When polydimethylsiloxane (PDMS) polymers contact clay minerals in the soil,
their siloxane backbone depolymerizes into lower-molecular-weight materials –
primarily Me2Si(OH)2. Depending on soil type, this
dimethylsilanediol will undergo further degradation:

Either in the soil via biodegradation

Or through evaporation into the atmosphere, where it will degrade
oxidatively via reaction with hydroxyl radicals

Whether degradation occurs in the soil or in the air, the result is the
same. The PDMS polymer breaks down into naturally occurring materials:

Silicates that exist naturally in the soil

Amorphous silica

Carbon dioxide

Water vapor

How PDMS enters the environment

PDMS fluids – Low-molecular-weight PDMS polymers are primarily used
in personal and household care products, like shampoos, conditioners, and
detergent antifoams. These products are rinsed away after use and end up in
municipal wastewater treatment plants. High-molecular-weight PDMS fluids used
as process aids, or surface treatments in industrial applications also end up
in wastewater treatment facilities.

High-molecular-weight PDMS molecules are virtually insoluble in water. This,
combined with their tendency to bind with organic material, effectively removes
them from the wastewater and onto the sludge during wastewater treatment. This
sludge is either used as a soil conditioner or it is incinerated. Either way,
the PDMS will break down into the same naturally occurring materials:
silicates, amorphous silica, carbon dioxide, and water vapor.

Solid silicones – Solid silicones such as PDMS-based rubbers or
sealants may be used in the home (as bathtub caulk or baby bottle nipples) or
in industrial applications, such as textile coatings, electronics protection
materials, moldmaking rubber, or rubber gaskets. They enter the environment as
a component of domestic or industrial waste, where they will be either
landfilled or incinerated. Many silicone rubber elastomers can be reclaimed and
recycled into new materials. This not only keeps them out of the landfill, but
extends their useful lives.